Lock down the prefix string now before it’s too late and document it. I see in Go that it’s lowercase ascii, which seems fine except for compound types (like “article-comment”). May be worth looking at allowing a single separator given that many complex projects (and ORMs) can’t avoid them.
The Go implementation has no tests. This is very unit-testable. Add tests goddammit!
For Go, I’d align with Googles UUID implementation, with proper parse functions and an internal byte array instead of strings. Strings are for rendering (and in your case, the prefix). Right now, it looks like the parsing is too permissive, and goes into generation mode if the suffix is empty. And the SplitN+index thing will panic if no underscores, no? Anyway, tests will tell.
As for the actual design decisions, I tried to poke holes but I fold! I think this strikes the sweet spot between the different tradeoffs. Well done!